Polycrystalline silicon has in recent years been used to a great extent as the interconnection material in integrated circuits. Polycrystalline silicon is desirable since it is very stable at high temperatures and since silicon dioxide can be chemically vapor deposited or thermally grown thereon. Polycrystalline silicon interconnections have been used in many types of integrated circuit applications such as charge-coupled device arrays, logic arrays, and one-device memory cell arrays.
An undesirable feature of polycrystalline silicon is its relatively high electrical resistance. Attempts to improve the performance of various integrated circuits by scaling down the device dimensions have not been entirely successful since the IR drops in the interconnections do not scale down while the voltage levels appropriate for operation are reduced. Therefore, it would be desirable to decrease the sheet resistance of the polycrystalline silicon interconnections in order to gain increased circuit speed.
It has been suggested that various refractory metals such as molybdenum and tungsten could be used in place of the polycrystalline silicon. However, these metals oxidize during chemical vapor deposition of the silicon dioxide, and since these oxides are much less stable than silicon dioxide, they pose a reliability problem with the finished integrated circuit.
In order to attempt to overcome the problem posed by employing such refractory metals alone, it has been suggested to replace some portion of the polycrystalline silicon layer with a layer of a silicide of certain metals. For instance, Rideout in IBM Technical Disclosure Bulletin, Volume 17, No. 6, November 1974, Reducing the Sheet Resistance of Polysilicon Lines in Integrated Circuits, pages 1831-33 suggested the use of hafnium silicide obtained by depositing hafnium on top of polycrystalline silicon and then heating to react the hafnium and polycrystalline silicon. In addition, Rideout suggested that other candidates for such purpose included, among others, tantalum silicide, tungsten silicide, and molybdenum silicide. Rideout further suggests that the lines can then be covered with chemically vapor deposited oxide.
It was also suggested in U.S. Pat. No. 3,381,182 to Thornton to form molybdenum silicide over polycrystalline silicon by methods similar to that disclosed by Rideout or by chemical vapor deposition through hydrogen reduction of molybdenum chloride and silane. Further discussions of forming various silicides including tungsten silicide by sputtering tungsten onto a silicon containing substrate and then heating to cause formation of the silicide can be found in French Pat. No. 2,250,198 and in Journal Electrochemical Society, Solid-State Science and Technology-Fabrication and Thermal Stability of W-Si Ohmic Contacts by V. Kumar, February 1975, pages 262-69.
However, the sputtering techniques suggested suffer from a number of disadvantages. In particular, the ability to accurately vary the silicide composition is quite restrictive. Also, when employing sputtering techniques, it is necessary to employ etching to remove metal from those desired areas which are not to include the silicide.
Accordingly, it is an object of the present invention to provide a method for forming silicides of certain refractory metals which is capable of accurately controlling and varying the composition of the silicide. It is a further object of the present invention to provide a process whereby the silicide can be removed from desired areas on the substrate by simple lift-off methods employing a solvent as opposed to the more complex etching procedures which require further masking.